Grating lobe

A grating lobe is an additional principal maximum caused by spatial sampling in a phased array. A regularly spaced linear array steered towards ${\theta }_0$ also has maxima wherever $\sin \theta =\sin {\theta }_0+m\lambda /d$ lies in $[-1,1]$. Taking $d<\lambda /2$ excludes such maxima for all steering angles; $d=\lambda /2$ is the usual limiting choice, with an endfire degeneracy at the extreme scan directions.